1. Field of the Invention
The present invention relates to an ophthalmologic apparatus used in an ophthalmologic clinic and the like.
2. Related Background Art
In recent years, a large number of apparatuses using image pickup elements have been developed. Even in ophthalmologic image taking apparatuses, ophthalmologic apparatuses in which an image of an eye to be examined is obtained as an electronic image using image pickup elements instead of a conventional silver-halide film have been developed to conduct electronic filing, a remote diagnosis, a diagnostic support using a computer, and the like.
According to such an ophthalmologic apparatus, for example, in the case of a non-mydriatic eye fundus camera, focusing and alignment adjustment are conducted using infrared light by a monochrome camera at the time of observation. In addition, a still image is picked up in synchronization with strobe light using another color camera at the time of image taking.
However, in recent years, an eye fundus camera comes to be used which includes not respective special image pickup elements for infrared observation and color still image taking which are used in a conventional apparatus but a common image pickup element.
When the common image pickup element is used for observation and image taking, a light source for observation using infrared light is different from a light source for color still image taking using strobe light. Therefore, in order to obtain suitable images at the observation and the image taking, it is necessary to set respective different gains.
For example, as disclosed in Japanese Patent Application Laid-Open No. 04-038582, there has been a method in which a gain of a camera is increased at the time of observation with low illumination to improve a contrast in an image and a gain of a camera is reduced at the time of image taking with high illumination to improve an S/N ratio of a taken image.
Thus, according to the above-mentioned conventional example, the gain of the observation camera and the amount of light from the observation light source are adjusted to improve the contrast in the observation image. However, when the gain and the amount of light are simply changed, a contrast in the entire image including an aperture region is enhanced. Therefore, it is impossible to enhance a contrast in only a region of interest such as a blood vessel which becomes an index in focusing and alignment adjustment. Accordingly, a sufficient contrast cannot be obtained by only adjusting the gain and the amount of light, so that there is the case where it is difficult to conduct focusing and alignment adjustment.
FIG. 4 is a histogram of the entire image in observation. The abscissa indicates brightness and the ordinate indicates a frequency. In FIG. 4, brightness levels of an aperture region are distributed in a range “A”, brightness levels of the entire eye fundus are distributed in a range “B”, and brightness levels of blood vessels near an optic disk portion which are used for focusing and alignment adjustment are distributed in a range “C”.
When infrared light is used as observation light, the reflection from not a retina but a choroid becomes dominant, so that a range in which image data of the region of interest on the retina are distributed is extremely narrow as shown in FIG. 4. Therefore, the contrast in the region of interest cannot be almost improved by only a linear correction such as the adjustment for the gain of the camera and the amount of observation light.
On the other hand, in the case of still image taking using strobe light, a correction method is desired which is different from a correction at the time of observation such as a gamma correction for monitor or a linear gain correction. Therefore, in order to improve the contrast in the observation image, a special correction circuit for observation is required as disclosed in Japanese Patent Application Laid-Open No. 05-199998. When such a special correction circuit is used, the entire circuit is upsized and complicated, thereby increasing a cost.